a. Field of the Invention
This invention relates generally to a circuit for controlling the amount of energy delivered to a load. More specifically, the invention relates to electrical energy supply for devices or apparatus which do not require fixed regulated voltages. An example of such a device is a thermal recording system which includes a plurality of heat-producing elements arranged in the form of an array, where each heat-producing elements requires a predetermined amount of energy, regardless of voltage level, to form a mark of predetermined quality or contrast, on a thermal-sensitive recording medium.
b. Description of the Prior Art
A wide variety of electrical devices and machines employ regulated power supplies to accurately control the energy actually delivered to a known load. Generally, a predetermined constant voltage is provided for a predetermined fixed period of time in devices requiring pulsed energy. In other devices where constant power is needed, an accurately predetermined voltage is constantly supplied. If power to a load is regulated for a predetermined amount of time then energy delivered to that load is also being controlled. The voltage parameter of conventional power supplies is generally regulated to remain constant.
Regulated power supplies, however, add cost and bulk to electrical equipment, in that relatively bulky load capacitors and other circuit components involved in voltage regulation are necessary.
Furthermore, it is important to match maximum power requirements of a load with the power output of a conventional power supply to ensure that the load will receive sufficient power. The cost of the power supply's regulating circuitry and its weight increase, however, with increasing power output. Product design restrictions usually necessitate consideration of average power requirements of the load in order to minimize the cost and weight of the power supply. The potential savings in power supply cost and weight are generally traded against the performance of the product.
In the particular field of thermal printing systems, information is "printed" on a thermal-sensitive recording medium through the use of a conventional thermal printhead. The thermal printhead generally includes a plurality of electrically resistive elements or "dots" arranged in a linear array. A printhead array may typically be made up of as many as 1600 or more individual resistive dots. Each dot is a fixed resistor which converts delivered electrical energy into heat energy. The heat energy causes nearby heat-sensitive paper to react so as to create a corresponding dark dot on the recording medium. These dots form on an advancing sheet medium a predetermined alphanumeric character or graphic symbol, as is well known in the art.
Such resistor dots may have a typical average power rating of approximately one watt per dot. The maximum power required by the entire printhead array in the case of a 1600 dot array, would be 1600 watts peak, if the entire array is to be energized at once. This would require a 1600 watt peak power supply to ensure that each dot could operate successfully. Due to the cost and bulk of certain electrical components, such as load capacitors, commonly found in conventional voltage regulating circuitry, the cost of a 1600 watt peak power supply might approach or surpass the intended cost of the entire thermal printer. Also, a 1600 watt peak power supply would be larger than the typical size of conventional thermal printers.
A currently conventional solution to the aforementioned problem is to provide, in a thermal printer, a regulated power supply which is rated for the average power requirements of the dot array, for example 200 watts. However, the 200 watt power supply could conceivably supply 400 watts of peak power (reserved capacitance) but would be incapable of providing power to address the 1600 watt peak load requirement. Thus, the printhead array is divided into several (e.g., four or more) segments, the printer circuitry then being designed to energize the four segments separately and staggered sequentially in some fashion. The heat-sensitive paper-recording medium be held stationary for the four energy pulse periods in this case. Thus it can be seen that the conventional power supply directly limits the speed of thermal printer operation.
Another solution is disclosed in U.S. Pat. No. 4,684,959, issued Aug. 4, 1987, to M. Mori et al., and assigned to Ricoh Company, Ltd. Broadly, Mori et al. disclose a power supply which includes a transformer having its primary side connected to an alternating current (a.c.) line, a rectifying diode connected across the secondary side of the transformer and a capacitor connected in parallel with the diode. Mori et al. state that with such a structure, the pulse width of an activation pulse to be applied to on of the resistors of the thermal printhead is controlled to maintain the product of (detected voltage) squared times (pulse width of applied pulse) substantially constant while monitoring and detecting the voltage to be supplied t the resistive dots for recording.
The system of Mori et al. ('959), while an improvement over previous systems, does not eliminate the use of load capacitance (see, e.g., col. 3, 11. 23, 32-33, 37, 43), often the bulkiest and most expensive component in power regulating circuitry. Although not "regulation" in the ordinary sense, Mori et al. nevertheless teach the use of a load capacitor 13 for smoothing the rectified line voltage. In so doing, the true sinusoidal a.c. (or rectified half-sinusoidal) waveform is eliminated, and accuracy of the energy control system deteriorates. One important cause of this is Mori et al.'s use of an average applied voltage rather than the actual detected applied voltage. As stated at col. 4, 11. 32-38, accuracy of delivered energy is maintained only within +/-5 percent, but this accuracy depends on properly matching the energy capacity of capacitor 13 to the known load condition. Thus, the Mori et al. accuracy is load sensitive. This use of a load capacitor presents the same drawbacks as the previously described prior art configuration wherein the resistor dot array of a thermal printer is divided and sequentially energized (including a modest load capacitance).
In the present invention, a precisely predetermined amount of energy is provided to a load without regulating or smoothing the voltage applied across the load in any way. The need for conventional regulating circuitry components is eliminated, resulting in an efficient, low cost, lightweight energy controller.